The production of oil and gas at offshore sites has increased dramatically in recent years. The drilling of wells at these sites is normally conducted from floating drilling vessels, such as semi-submersibles or drill ships, through a riser that extends from the vessel to a subsea installation at the site. A riser entry/reentry system is used to make the connection between the vessel and the subsea installation. During drilling, the vessel is maintained in position above the subsea installation by means of either a static or dynamic positioning system.
One of the problems that can occur, however, with the dynamic positioning system is that of "drift-off". That is, power to the vessel's positioning thrusters or control of the thrusters is lost, causing the vessel to drift off location. Another but similar problem is "drive-off". Here, the vessel is forced off location because an incorrect control command is sent to the vessel's thrusters. In both situations, obviously, the riser has to be disconnected from the subsea installation. Accordingly, the riser entry/reentry system, a guidelineless one being normally employed when operating from a dynamically positioned vessel, must permit quick release of the riser from the subsea installation. The system should also allow the riser to be reconnected to the subsea installation with very little loss of time and with very little difficulty.
In certain waters, icebergs also pose a hazard to drilling operations. For should the drilling site be in the path of an oncoming iceberg, drilling operations will have to be quickly terminated so that the vessel may move out of the way. Here again, and this applies whether the vessel is dynamically or statically positioned above the drilling site, there must be the capability to quickly disconnect the riser from the subsea installation. And when the vessel moves back on location, or moves on location for the first time, the riser entry/reentry system should permit the riser to be connected to the subsea installation with a minimum of problems.
U.S. Pat. No. 4,167,215, issued Sept. 11, 1979, is a guidelineless system for entry and reentry into a subsea wellhead. This system employs a TV/sonar guidance means for locating the wellhead along with additional apparatus for properly positioning a riser and service equipment at the wellhead. The system has an offset funnel mounted on the wellhead to guide a probe, which is mounted at the lower end of the riser, into alignment with the wellhead. When the probe is located in the funnel's neck, it is locked therein so that proper alignment is maintained between the wellhead and the riser as well as the associated service equipment. With the probe locked into position in the funnel neck, well entry, tree installation, well reentry, tree cap removal and replacement, and downhole wireline operations may be completed without the use of guidelines. To perform repair and replacement work at the wellhead, the system further includes a component manipulator device which is used to maneuver equipment about the wellhead. The manipulator includes a pair of hydraulic lowering arms that are connected to a running tool; the hydraulic arms are also connected to the probe. The hydraulic arms are used to position the running tool and to lower the probe into the offset funnel.
The above-discussed system, however, is an overly complex arrangement for a riser entry/reentry system that is designed simply to permit quick release of the riser from the subsea installation and quick reconnection thereto. Moreover, since the probe of this system is locked into the neck of the offset funnel, it does not actually provide a quick release capability. This is due to the fact that before the riser could be disconnected from the subsea installation, the probe would have to be unlocked and withdrawn from the funnel. And in high angle release situations that occur during vessel "drive-off" or "drift-off", the probe would hang up in the funnel, delaying removal of the riser from the subsea installation and causing damage to the riser and subsea installation.
The present invention is thus directed to a rather simple guidelineless riser entry/reentry system that permits a safe and quick release of the riser from the subsea installation. The system also provides an efficient means for riser entry and reentry. And the system can also be used to convert a substantial guideline system into a guidelineless one while still retaining the features of the guideline system.